Understanding $.proxy() in jQuery

Understanding $.proxy() in jQuery

From docs I understand that .proxy() would change the scope of the function passed as an argument. Could someone please explain me this better? Why should we do this?

Solutions/Answers:

Solution 1:

What it ultimately does is it ensures that the value of this in a function will be the value you desire.

A common example is in a setTimeout that takes place inside a click handler.

Take this:

$('#myElement').click(function() {
        // In this function, "this" is our DOM element.
    $(this).addClass('aNewClass');
});

The intention is simple enough. When myElement is clicked, it should get the class aNewClass. Inside the handler this represents the element that was clicked.

But what if we wanted a short delay before adding the class? We might use a setTimeout to accomplish it, but the trouble is that whatever function we give to setTimeout, the value of this inside that function will be window instead of our element.

$('#myElement').click(function() {
    setTimeout(function() {
          // Problem! In this function "this" is not our element!
        $(this).addClass('aNewClass');
    }, 1000);
});

So what we can do instead, is to call $.proxy(), sending it the function and the value we want to assign to this, and it will return a function that will retain that value.

$('#myElement').click(function() {
   // ------------------v--------give $.proxy our function,
    setTimeout($.proxy(function() {
        $(this).addClass('aNewClass');  // Now "this" is again our element
    }, this), 1000);
   // ---^--------------and tell it that we want our DOM element to be the
   //                      value of "this" in the function
});

So after we gave $.proxy() the function, and the value we want for this, it returned a function that will ensure that this is properly set.

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How does it do it? It just returns an anonymous function that calls our function using the .apply() method, which lets it explicitly set the value of this.

A simplified look at the function that is returned may look like:

function() {
    // v--------func is the function we gave to $.proxy
    func.apply( ctx );
    // ----------^------ ctx is the value we wanted for "this" (our DOM element)
}

So this anonymous function is given to setTimeout, and all it does is execute our original function with the proper this context.

Solution 2:

Without going into greater detail (which would be necessary because this is about Context in ECMAScript, the this context variable etc.)

There are three different types of “Contexts” in ECMA-/Javascript:

  • The global context
  • Function context
  • eval context

Every code is executed in its execution context. There is one global context and there can be many instances of function (and eval) contexts. Now the interesting part:

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Every call of a function enters the function execution context. An execution context of a function looks like:

The Activation Object
Scope Chain
this value

So the this value is a special object which is related with the execution context. There are two functions in ECMA-/Javascript which may change the this value in a function execution context:

.call()
.apply()

If we have a function foobar() we can change the this value by calling:

foobar.call({test: 5});

Now we could access in foobar the object we passed in:

function foobar() { 
    this.test // === 5
}

This is exactly what jQuery.proxy() does. It takes a function and context (which is nothing else than an object) and links the function by invoking .call() or .apply() and returns that new function.

Solution 3:

I have written this function:

function my_proxy (func,obj)
{
    if (typeof(func)!="function")
        return;

    // If obj is empty or another set another object 
    if (!obj) obj=this;

    return function () { return func.apply(obj,arguments); }
}

Solution 4:

The same goal can be achieved using a “Immediately-Invoked Function Expression, short: IIFE” self executing function:

    $('#myElement').click(function() {  
      (function(el){
         setTimeout(function() {
              // Problem! In this function "this" is not our element!
            el.addClass('colorme');
        }, 1000);
      })($(this)); // self executing function   
    });
.colorme{
  color:red;
  font-size:20px;
}
<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
<script src="https://code.jquery.com/jquery-3.1.0.js"></script>

  <div id="myElement">Click me</div>
</body>
</html>